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Mysterious Pond water loss

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I have a backyard pond, with common goldfish. It is well maintained, with a small knee high fence around it, which is completely covered by a net canopy staked to the ground. It is currently winterized, with one small trickle fountain filter in the center, and a pond de-icer which was turned on as the nights here are now getting below 32 degrees. The de-icer is also staked to a deep submerged potted plant, so it will not run into the sides of the liner. The pond liner is a hard molded liner.

This is not a recent problem, but a mysterious one that makes no sense, its happened all summer, and now, even that it is winterized and covered with netting. Here's the problem.

Pond runs fine, for weeks, water level stays the same, no equipment leaks, and except for an occasional bucket of water to be added for evaporation, its fine. Three weeks will go by, and then suddenly we wake up, and 1/3 to half of the water is mysteriously gone. We check it, and there's nothing amiss, no equipment leaks, and the fountain is just on a trickle so there is no splash out from that. We refill it, and it holds water just fine, go a week, two weeks, water level is fine, then bam...... wake up, a massive amount of water is gone.

I have a hard time believing the molded liner is leaking, as it holds water, for quite some time, in all temperatures, then just suddenly, pouf!

Anyone have any ideas what's causing this? Has anyone had a similar problem? I'm at my wits end!

Possible temperature shifts, water warming during the day and cooling during the night is causing a crack to to appear and let the water leak out into the ground? Really sounds like a leak in the liner to me, might have to empty the pond completely and check.

Does the amount of water that disappears always go down to the same level? That might indicate the level of the crack in the liner if there is one.

The other unlikely and more amusing answer would be, are there any animals like deer or something that are using the pond as a water source?:p

I'm not sure, as I said, temperature seems to make no difference. Did it in the summer, and doing it now, it makes no sense. temp will fluctuate, and water level stays fine, then just, out of no where, boom, half the pond water is gone. It does tend to drain to the same level every time. I did order a flexible pond liner to line this with, just to be on the safe side.

I'm just at a loss as to where its going, and what kind of leak has a temperament like this!

Maybe drain the pond down a bit further than the level it usually drains itself to, and take a look at the lining at that level. At a loss of what else to suggest. Let us know if you discover what is happening, would be interested to find out.

Any plumbing that is running outside of the pond. Temperature could cause seals to move letting air in and or water out as materials expand and contract with temperature. Or other force (gravity) over time that is a result of the weight of the water/plumbing. Although I would expect you to see that leaked water in some form by walking around the pond with soaking wet ground.

In terms of temperament. Temperature contraction is one possibility but one thing that occurs to me is if the leak is occasionally stopped by the pressure of an air bubble and that pressure is changed but soil / ground / material movement due to pressure / temperature change.

What is under the pond liner, how well does that soil / support drain? Is the soil wetter around the pond than the rest of the garden perhaps?

Otherwise the question is who or what is living in your pond and where are they going?